A known technique of increasing information throughput in synchronous signalling systems is to use increased bit rates to communicate the information. Older wide area radio communication systems, such as systems using the POCSAG (Post Office Commission Standards Advisory Group) signalling protocol, were designed to operate with pagers having a fixed bit rate. Newer systems, such as those using the FLEX.TM. family of signalling protocols licensed by Motorola, Inc., of Schaumburg, Ill., allow the use of a bit rate selected from a set of predetermined bit rates, typically by a set of portable or mobile receiving units operating in a common transmission cell.
The POCSAG and FLEX.TM. signalling protocols, like other signalling protocols, are defined using a hierarchical signalling protocol, with several protocol divisions of differing levels, wherein each protocol division is formed by a plurality of protocol divisions of a next lower level. The protocol divisions can be further identified as protocol divisions of the first, second, etc., level, with the first level protocol division being a POCSAG frame in POCSAG, comprising two POCSAG words and a FLEX.TM. block in FLEX.TM., comprising 8, 16, or 32 FLEX.TM. words, depending on the bit rate, so that FLEX.TM. blocks are of the same duration regardless of bit rate. The second level protocol division in POCSAG is a POCSAG batch, comprising eight POCSAG frames; the second level protocol division in FLEX.TM. is a FLEX.TM. frame, comprising 11 FLEX.TM. blocks. In POCSAG, no higher level protocol divisions are formally named; at this level, a single POCSAG transmission comprises any number of POCSAG batches, each preceded by a synchronization code. The third level protocol division in FLEX.TM. is a FLEX.TM. cycle, comprising 128 FLEX.TM. frames. The fourth level protocol division in FLEX.TM. is an hour, which comprises 15 FLEX.TM. cycles and has a duration of sixty minutes. Protocol division of higher levels in FLEX.TM. are defined by standard time divisions (day, month, and year).
As stated above, a bit rate used in the FLEX.TM. signalling protocol can be one of several predetermined bit rates. The bit rate being used for subsequent information being transmitted within a FLEX.TM. frame is identified at the beginning of each FLEX.TM. frame, during a synchronization pattern which is provided to allow any radios, such as pagers, that have been not receiving for a FLEX.TM. cycle or more to acquire exact word and bit synchronization. Pagers and other radios that are active in a FLEX.TM. system can be set up to go into a battery saving mode for all frames except one in a FLEX.TM. cycle, or can alternatively be set up to go into an active receiving mode for a plurality of frames, such as two or four in each cycle, effectively reducing an average latency for receiving messages, but at a trade-off of poorer battery life than when only one frame per cycle is used. Thus, the frame can alternatively be described as a battery saving protocol division in FLEX.TM..
Each time a radio goes into the active receiving mode, it acquires exact synchronization and determines the bit rate to be used within the FLEX.TM. frame. It further determines a protocol position within a frame at which information for the specific radio is located, for example, a frame number, a block number and a word number. The frame number could be the same frame at which the radio went into the active mode, or any other frame.
This design provides an efficient approach to operating a group of radios which receive message information within the same FLEX.TM. frame at a common bit rate which is maximized for the group. Generally, this works well when a common bit rate is used by a group of radios which always receive at one of the predetermined bit rates; the radios in the group can be assigned to a certain frame or frames that are predeterminedly sent using the bit rate. Thus, a maximum bit rate can be chosen for the group based on the capability of the system (for example, the density and power of fixed system transmitters) and the sensitivity of the receiving radios.
A third signalling protocol is described in U.S. Pat. No. 5,613,194 issued Mar. 18, 1997 to Olds et al., entitled "SATELLITE-BASED CELLULAR MESSAGING SYSTEM AND METHOD OF OPERATION THEREOF," in which the bit rate used for communicating information to pagers is fixed. Although this system is quite efficient, it does not provide for optimization of the bit rate as determined for a particular link to a particular receiving radio.
When all radios in a group of radios are of the same sensitivity and operate within a communication cell associated with a transmitter, it is well known to one of ordinary skill in the art that a specific link to one radio can be substantially better or worse than a link to another radio in the same group. Further improvements in throughput efficiency would be possible if a bit rate could be conveniently established for transmitting a portion of a message (up to and including a complete message) to each radio for which a portion of a message is queued depending on the characteristics of a specific link to the one radio.
Thus, what is needed is an improved technique within a radio communication system for establishing an optimum bit rate for transmitting a portion of a message to a radio which is compatible with a battery saving scheme, and a corresponding technique in a radio for responding to the optimum bit rate.